Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

VCO linearization question!

Status
Not open for further replies.

hawsona

Junior Member level 1
Joined
Jan 22, 2002
Messages
16
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
192
linearize vco

Now I have to linearize the VCO output frequency! Use digital method or
RF method!
but I don't know how to change the VCO schemtic to make VCO linearizing!
I know one method is use back to back varactor! But it's not very ideal!

Can any one talk with me for some RF design question?
I will very glad to contact with you!

my mail: carmine_rene@hotmail.com
 

HBK

Member level 2
Joined
Apr 18, 2002
Messages
51
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Activity points
242
vidmar vco

How abt a ring oscillator?
 

FANT

Advanced Member level 1
Joined
Feb 27, 2002
Messages
415
Helped
52
Reputation
104
Reaction score
37
Trophy points
1,308
Location
Italy
Activity points
3,646
transistor reactance modulator

I have had the same problem with a VCO working in the low UHF band and i have solved it usin a linerizing diode like a 1N4148.
The result is good because on a swing of about 10 % of the center frequency the deviation change no more than 2 %.
Otherwise use ( if the frequency is much higher ) the VCO used by Matjaz Vidmar as 1st LO on his spectrum analyzer ( he use 6 varicap ! ).
I have build it and is very linear.
If anybody can tellme how to upload a schematic ( *.jpeg, for example ) i will send you the one i have used and the one of Vidmar.
Bye.
 

mike_bihan

Full Member level 3
Joined
Mar 21, 2002
Messages
185
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,296
Activity points
1,544
reactance modulator with bjt

would you like to share the schematic with me?
or, it will be helpful if you can tell me the name of the paper described.

Thank you!
han.bi@sina.com
 

flatulent

Advanced Member level 5
Joined
Jul 19, 2002
Messages
4,629
Helped
489
Reputation
980
Reaction score
150
Trophy points
1,343
Location
Middle Earth
Activity points
46,689
linearized varicap

The back to back varicaps are for improving the phase noise by keeping the diodes from becoming forward biased over the rf cycle.

The linearization methods preceed the varicap by a network with a nonlinear in-out characteristic. This can be done with many methods, such as using diodes.

One problem with this is the extra phase shift through the network which must be taken into acount if the vco is being used in a PLL.

Another method if you are modulating is to demodulate the signal and apply feedback to your audio input stage from the demodulator.
 

g579

Full Member level 6
Joined
Dec 31, 1999
Messages
322
Helped
19
Reputation
38
Reaction score
4
Trophy points
1,298
Location
Somewhere
Activity points
2,470
linearizing varicap

About the question of using analog or digital linearization.
If the VCO control signal is a slow one (or a narrow band PLL) the easiest way is using diode linearizer with op.amp.
Digital linearization is simple too and can be used in slow anf fast cases too. With fast A/D converter to the VCO sweep and an EEPROM lookup table you can switch (regulate) the gain of some aplifier circuit, carriing out the linearization.
 

strabush

Member level 5
Joined
Apr 10, 2002
Messages
84
Helped
8
Reputation
16
Reaction score
3
Trophy points
1,288
Activity points
690
opamp linearizer

From my experience, if you use a hyperabrupt varactor (or two in series) as the ONLY capacitor in the tank LC resonator, you will get a constant frequency / control voltage ratio, for a wide frequency range. In order to implement such a circuit you need a circuit that the feedback is done through the inductive part of the tank, like a Hartley oscillator topology.
 

edf

Full Member level 2
Joined
Feb 21, 2002
Messages
121
Helped
21
Reputation
42
Reaction score
18
Trophy points
1,298
Activity points
700
linearizing vco

I have designed a VCO covering 650-1100MHz in Colpits configuration with varactor-inductor on base and second varactor in emitter that had good linearity. The emitter varactor was in series with fixed value and only affected low end of tuning range.
 

g579

Full Member level 6
Joined
Dec 31, 1999
Messages
322
Helped
19
Reputation
38
Reaction score
4
Trophy points
1,298
Location
Somewhere
Activity points
2,470
tube voltage control oscillator

strabush and edf both are right too.
These things may also give you a good enough result.
The freq - voltage characteristics can be modeled by EDA tools (even linear simulation may give acceptable results)
 

yingyang

Full Member level 2
Joined
Dec 26, 2002
Messages
126
Helped
2
Reputation
4
Reaction score
2
Trophy points
1,298
Activity points
862
linearization circuit diode

Can VCO be implemented without a diode or varactor????
 

strabush

Member level 5
Joined
Apr 10, 2002
Messages
84
Helped
8
Reputation
16
Reaction score
3
Trophy points
1,288
Activity points
690
linearization of varactors

Attached is an article about controlling the frequency of an oscillator without varactors:
 

g579

Full Member level 6
Joined
Dec 31, 1999
Messages
322
Helped
19
Reputation
38
Reaction score
4
Trophy points
1,298
Location
Somewhere
Activity points
2,470
vidmar - spectrum analyzer update

For smaller freq. changes sometimes changing the CB capacitance of a BJT transistor (by changing the collector voltage) may result a VCO without varicap.
 

edf

Full Member level 2
Joined
Feb 21, 2002
Messages
121
Helped
21
Reputation
42
Reaction score
18
Trophy points
1,298
Activity points
700
varactor linearization

It is possible to design varactoless VCO's that can be tuned over a 15% bandwidth in the VHF range if you use a transistor designed for forward AGC such as the MPSH30 or 31. The B-E capacitance is current controlled. Use colpitts without lumped cap across B-E for max deviation.
 

zorro

Advanced Member level 4
Joined
Sep 6, 2001
Messages
1,131
Helped
356
Reputation
710
Reaction score
298
Trophy points
1,363
Location
Argentina
Activity points
8,902
rf linearizer

yingyang said:
Can VCO be implemented without a diode or varactor????
A very effective and old method is the reactance modulator, called also reactance tube or reactance valve because it comes from the epoch of vacuum tubes.
Search for it.

The idea is:
Imagine an ideal bipolar transistor (it have to be polarized, of course) with an admittance Ybc between B and C, and an Ybe between B and E, and Ybe>>Ybc. The impedance presented by a transistor between C and E is Yeff=Gm*Ybc/Ybe, where Gm is the transconductance of the transistor.

The transconductance depends on its collector current, so varying Ic you can control Yeff.
Put a capacitor between B and C and a resistor between B and E (R<<omega*C). You have a variable capacitor.

(You could make a variable inductor as well changing R with C or putting L instead of C, but always Ybe>>Ybc. The combination given above is the most practical).
Regards

Z
 

yingyang

Full Member level 2
Joined
Dec 26, 2002
Messages
126
Helped
2
Reputation
4
Reaction score
2
Trophy points
1,298
Activity points
862
oscillatoer linearizer circuit

Thanks pal for the idea.

However, I am looking for techniques in RF and microwave frequency range. In addition, 1 am looking for a bandwidth of about 40%.

Any idea???

:oops: :oops: :oops: :oops:
 

zorro

Advanced Member level 4
Joined
Sep 6, 2001
Messages
1,131
Helped
356
Reputation
710
Reaction score
298
Trophy points
1,363
Location
Argentina
Activity points
8,902
question and solution vco

Yingyang, You need a huge fractional bandwidth.
You can modulate narrow-band FM and then apply multiply-and shift method: frequency multiplying multiplies the frequency deviation, and down-conversion conserves it, increasing the fractional deviation.
Regards

Z
 

g579

Full Member level 6
Joined
Dec 31, 1999
Messages
322
Helped
19
Reputation
38
Reaction score
4
Trophy points
1,298
Location
Somewhere
Activity points
2,470
linearization opamp diode

yingyang,

If price is not the major concern YIG (Yttrium - Iron - Garnet) oscillator may be a solution. Every handbook on MW has references about that.
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top